|Bioenergy Primer: Modernised Biomass Energy for Sustainable Development (UNDP, 2000, 153 p.)|
|Chapter 4. Environmental Issues|
Although biomass is usually called a "renewable" source of energy, this term is used somewhat loosely, as biomass production typically requires the use of fossil fuels. How much fossil fuel is used depends on the particular form of biomass and varies dramatically among biomass options. Usually, the degree of fossil fuel consumption is measured in terms of an energy ratio: the energy of the biomass produced divided by the energy of the fossil fuel consumed.3 To be accurate, the amount of fossil fuel consumed should include all inputs into the biomass feedstock:
3 Some studies report the "net energy ratio" (the amount of bioenergy produced minus the amount of fossil fuel consumed, divided by the amount of fossil fuel consumed). Numerically, the net energy ratio simply equals the energy ratio minus one.
· fuels consumed by farm machinery in land preparation, planting, tending, and harvesting;
· fossil feedstocks used to produce chemical inputs such as herbicides, pesticides, and especially fertilisers, which are energy intensive;
· energy used for irrigation, if any;
· fuels consumed during transport, storage, and processing of the biomass; and
· the energy required to manufacture and transport equipment and machinery (which is often neglected because it is a relatively minor contribution - a few percent of the energy content of the biomass for transport distances within 200 km (Borjesson, 1996a, b).
Although biomass is usually called a "renewable" source of energy, this term is used somewhat loosely, as biomass production typically requires the use of fossil fuels. How much fossil fuel is used depends on the particular form of biomass and varies dramatically among biomass options.
A single quantitative measure such as the energy ratio can be a misleading comparison among biomass options, however, because it does not reflect the non-energy agricultural outputs, which can be as important as the energy outputs in terms of their economic, social, or environmental value. Nonetheless, for comparing bioenergy cycles that provide comparable outputs, such quantitative measures reasonably reflect the relative reliance on fossil fuels.
Many agricultural or forestry residues can be considered essentially renewable, because negligible fossil fuel is consumed in addition to what is required to produce the primary crop. For purpose-grown crops, energy ratios are generally higher for perennial crops than for annual crops, which are more energy intensive because they involve greater use of machinery and a higher level of chemical inputs. Nevertheless, some annual crops are preferable for other reasons, e.g., less land might be needed per unit of energy produced as a result of higher yields, or a particular agricultural activity may have strong political support. For example, production of corn-based ethanol in the United States has an energy ratio about one (Wyman et al., 1993), but enjoys strong political support in the form of subsidies to producers.
For many perennial energy crops, energy ratios for feedstock production are high enough to make them attractive energy options. For example, some crops (poplar, sorghum, and switchgrass) grown in a temperate climate have energy ratios of 12 to 16 (Turhollow and Perlack, 1991). In tropical climates with good rainfall, these ratios could be considerably higher, due to both higher yields and less energy-intensive (i.e., more labor-intensive) agricultural practices (Ravindranath and Hall, 1995).